This invention relates to a resist material for forming a resist pattern by application of radiation such as electron beams, X-rays and ultraviolet rays.
Many attempts have been made for increasing the integration scale of semiconductor integrated circuits in recent years, and as one of the significant outcomes of such attempts, there has been proposed a so-called etching method according to which method a fine and minute pattern can be obtained on the substrate by utilizing radiation with short wave length such as electron beams, X-rays and ultraviolet rays.
This invention is to provide a resist material for use in such etching method utilizing the above-said types of radiation. The resist material provided according to this invention is of the positive type which is solubilized in the developing solvent by application of said radiation.
Poly(methylmethacrylate) (hereinafter referred to as PMMA), crosslinked PMMA and poly(styrene sulfone) are known as typical examples of the conventional positive type resist materials, but all of these materials are unsatisfactory in respect of sensitivity. For example, the minimum required incident charge of electron beams is as high as 5 .times. 10.sup.-5 C/cm.sup.2 in the case of PMMA, 0.8 .times. 10.sup.-5 C/cm .sup.2 in the case of crosslinked PMMA, and 1 .times. 10.sup.-5 C/cm.sup.2 in the case of poly(styrene sulfone), and for drawing, for example, a 3-inch square mask pattern, it is necessary to apply fairly intense electron beams for a prolonged period of time which would amount to the order of days.
Use of a polymer of poly(dimethacryl imide) or a copolymer thereof with a vinyl monomer as a positive resist material has been proposed recently (U.S. Pat. No. 3,964,908). The resist material provided according to this proposal is composed of a polymer having the following structural unit: ##STR1## with the C/N atomic ratio of 8, or a copolymer consisting of said polymer and a vinyl monomer such as styrene or methacrylic acid ester. However, in this proposal, too, a fairly high electron beam incident charge, on the order of 10.sup.-5 coulomb/cm.sup.2, is required for forming an image. This level of incident charge is only slightly better than those required for the conventional PMMA resists.
The primary object of this invention is to provide a resist material having sensitivity to the electron beams with an incident charge of a level which is one to two figures lower than that required for the conventional resist materials.
Another object of this invention is to provide a resist film having high heat stability sufficient to withstand ion beam etching.
The resist provided according to the present invention is composed principally of polymethacrylamide (hereinafter referred to as PMAA) or a copolymer thereof and has a sensitivity to electron beam irradiation of the minimum incident charge of 2 .times. 10.sup.-7 C/cm.sup.2. This resist requires short-time irradiation and has excellent thermal stability to well withstand ion beam etching.